How it works
Two measurements and your block size give you the whole shopping list.
Measure the wall
Length along the ground and height above the finished grade, in feet.
Enter your block
The face length and height printed on the pallet tag. 12 × 4 in is the most common small wall block.
Get the full list
Blocks, caps, gravel for the base and drainage, and drain pipe.
How to estimate a block retaining wall
Block count is simple: the number of courses times the blocks in each course. Courses come from the height — a 3 ft wall with 4 in blocks is nine courses above ground — plus one buried course below grade, which keeps the base from kicking out. Blocks per course come from the length: a 20 ft wall with 12 in blocks is twenty per course. Ten courses of twenty is 200 blocks, and 5% extra for cut ends and a broken corner brings you to 210.
The gravel is what holds it up
Most failed walls did not fail because of the blocks. They failed because water built up behind them. A segmental wall needs two kinds of gravel: a compacted base, about 6 in deep in a trench roughly twice the block depth, and a drainage column of clean crushed stone at least 12 in wide behind the blocks for the full height. A perforated drain pipe at the bottom of that column carries the water out the ends of the wall.
For the example wall that is 20 cubic feet of base and 60 of drainage stone, about 3 cubic yards or a little over 4 tons. Order it by the ton from a landscape yard; bagged gravel for a wall this size gets expensive fast.
When to call an engineer
Around 4 ft is the usual line. Above it — or if the wall holds back a driveway, a slope or a building — most jurisdictions require an engineered design with geogrid reinforcement and a permit. The calculator will still count your blocks, but treat the number as a budget, not a plan.
A worked example: a 30-foot garden wall
A wall 30 feet long and 2½ feet high, built from 12 × 4 inch face blocks, has 30 inches ÷ 4 = 7.5 courses above ground, rounded up to 8, plus one buried course: 9 courses. Each course takes 30 blocks, so with 5% extra the order is 284 blocks and 30 caps.
The stone is the bigger load. The base trench (2 feet wide, 6 inches deep) is 30 ft³, and the drainage column behind the wall (1 foot wide, full height) is 75 ft³: 3.89 cubic yards, or about 5.4 tons of crushed stone, plus 30 feet of perforated drain pipe. That is why the gravel order usually arrives by truck, even for a modest wall.
Heights and what they need
| Wall height | What is typical |
|---|---|
| Up to 2 ft | Buried course, compacted base, drainage stone and pipe |
| 2 to 4 ft | All of the above; geogrid often recommended by the block maker |
| Over about 4 ft | Usually an engineered design and a permit |
| Any height with a load above | Driveway, slope or building above: get a design |
Block makers publish design charts for their own systems, including the setback of each course and when geogrid is needed. Use the chart for the block you are buying.
Mistakes that make walls lean
- Backfilling with soil. The space behind the blocks must be clean crushed stone so water drains. Soil holds water, and water pressure pushes walls over.
- Skipping filter fabric. A layer of fabric between the drainage stone and the soil behind it stops fines from clogging the stone over the years.
- A level base that is not compacted. The first course has to be level and the base under it compacted; every error at the bottom grows as the wall rises.
- Curves without recounting. On a curved wall, measure the length along the face of the blocks. Outside and inside curves change how the blocks fit, and caps on curves often need cutting.
What it costs
Small segmental wall blocks commonly cost about $2 to $5 each in the US and caps about $3 to $7. For the example wall, blocks and caps come to roughly $658 to $1630, plus about 5.4 tons of stone, pipe, fabric and adhesive for the caps. Prices vary by style and region.
Frequently asked questions
How many blocks do I need for a 20 ft retaining wall?
A 20 ft wall 3 ft high, built with standard 12 × 4 in blocks, takes about 210 blocks: nine courses above grade, one buried course, and 5% extra for cuts and breakage, plus 20 cap blocks.
Why do you bury the first course?
A buried course keeps the base from sliding out and hides the leveling pad. Most block makers say to bury at least one course, or about 10% of the wall height, whichever is more.
How much gravel goes behind a retaining wall?
Plan on a 12 in wide column of clean gravel behind the blocks for the full height, plus a 6 in deep compacted base in a trench about 24 in wide. For a 20 × 3 ft wall that is about 3 cubic yards.
How tall can a block retaining wall be without an engineer?
Many codes and block manufacturers draw the line at about 4 ft. Taller walls, or walls holding back a slope, driveway or building, usually need an engineered design and a permit.
More material calculators
Estimates are for planning only. Walls over about 4 ft, or holding back a slope or driveway, usually need an engineered design and a permit.